松宮 久

This study challenged to improve the resistance against hydrogen embrittlement by increasing the concentration of carbon segregated at prior austenite grain boundary (PAGB), XPAGB, in low-carbon martensitic steels. The specimens with/without carbon segregation treatment (Non-seg and Seg specimens, respectively) had almost the same microstructure, other than higher XPAGB in the Seg specimen. While the uncharged Non-seg and Seg specimens exhibited similar mechanical properties, the maximum stress of the hydrogen-charged specimen was much higher in the Seg specimen than that in the Non-seg specimen even when diffusible hydrogen contents were almost the same. In addition, the fraction of intergranular fracture surface was much smaller in the Seg specimen. Based on these results, we conclude that the segregated carbon suppressed the accumulation of hydrogen around PAGB by site competition and increased cohesive energy of PAGB, leading to the significantly improved resistance against hydrogen-related intergranular fracture.

The present study investigated the hydrogen-related fatigue fracture under various test frequencies in low-carbon martensitic steel. In the hydrogen-charged specimen, although the number of cycles to failure decreased with decreasing test frequency, the time to failure was almost the same regardless of the test frequency. Observation of fracture surface revealed that the transgranular surface was a main component in the uncharged specimen, while the intergranular surface was often observed especially at the lower test frequency in the hydrogen-charged specimen. In addition, for the transgranular fracture, cracks often propagated across the laths regardless of test conditions. The high-strained region was observed over a relatively wide area in the uncharged specimen. On the other hand, the hydrogen-related fatigue-crack propagation was accompanied by intense localized plastic deformation, which could accelerate crack growth. The intergranular cracking and high localization of plastic deformation could be the possible reasons for decreasing the fatigue life by the presence of hydrogen.

A typical hydrogen-related transgranular fracture, namely quasi-cleavage fracture, is usually accompanied by serrated markings on the resultant fracture surfaces in steels with body-centered cubic phases. The present paper investigated the microscopic three-dimensional morphology and crystallographic feature of serrated markings in a 2Mn-0.1C steel mainly composed of ferrite microstructure. The serrated markings corresponded to the corners of the step-like morphologies which consisted of microscopic {011} facets whose longitudinal directions were almost parallel to < 110 > or < 112 > direction. In addition, the microscopic {011} quasi-cleavage facets had the largest inclination angle from tensile axis among six crystallographically equivalent {011} planes, suggesting that resolved normal stress imposed on the {011} plane is an important factor for the hydrogen-related quasi- cleavage fracture. We propose that not only the slip deformation enhanced by hydrogen but also the coalescence of vacancies / voids induced by hydrogen-enhanced plastic deformation should be considered for understanding the mechanism of the hydrogen-related quasi-cleavage fracture along the {011} planes.